1. Field of the Invention
The present invention relates to mechanisms for extracting water from a web of material, and, more particularly, from a fibrous web being processed into a paper product on a papermaking machine. Specifically, the present invention is an impermeable belt designed for use on a long nip press on a papermaking machine, and a method for constructing the impermeable belt.
2. Description of the Prior Art
During the papermaking process, a fibrous web is formed on a forming wire by depositing a fibrous slurry thereon. A large amount of water is drained from the slurry during this process, after which the newly formed web, left behind on top of the forming wire, proceeds to a press section. The press section includes a series of press nips, in which the fibrous web is subjected to compressive forces designed to remove additional water therefrom. The web finally proceeds to a drying section, which includes heated dryer drums around which the web is directed. The heated dryer drums reduce the water content of the web to a desirable level through evaporation, thereby completing the manufacture of a paper product.
Rising energy costs have made it increasingly desirable to remove as much water as possible from the web prior to its entering the dryer section. The dryer drums are typically heated from within by steam and related costs can be substantial, especially when a large amount of water needs to be removed from the web.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rollers. In recent years, the use of long press nips has been found to be advantageous over the use of nips formed by pairs of adjacent rollers. Long press nips are so called because they have a greater extent in the longitudinal, or machine, direction than those formed by pairs of adjacent rollers. The longer the web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less will remain to be removed through evaporation in the dryer section. This is accomplished in a long press nip by virtue of the fact that any given portion of the fibrous web takes a longer time to pass through the nip.
The present invention relates to long nip presses of the shoe type. In this variety of long nip press, the nip is formed between a cylindrical press roller and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roller. When roller and shoe are bought into close physical proximity, a nip is formed which can be five to ten times longer in the longitudinal, or machine, direction than one formed between two press rollers. This increases the so-called dwell time of the fibrous web in the long nip while maintaining the same level of pressure per square inch pressing force used in a two-roller press. The result of his new long nip technology has been a dramatic increase in dewatering of the fibrous web in the long nip when compared to conventional nips on paper machines.
A long nip press of the shoe type requires a special belt, such as that shown in U.S. Pat. No. 4,946,731 to Dutt. This belt is designed to protect the press fabric supporting, carrying, and dewatering the fibrous web from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt must be made with a smooth impervious surface that rides, or slides, over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric thereby subjecting the press fabric to minimal amounts of rubbing against stationary components of the press.
A typical configuration for a long nip press involves the use of two press fabrics. The components in the nip can be listed in the following sequence: a grooved cylindrical press roller, a first press fabric, the fibrous web or nascent paper sheet, a second press fabric, the special belt, the lubricating film, and the arcuate pressure shoe.
Many of the long nip presses currently in commercial operation are of the above configuration. However, this so-called double-felted long nip press is limited to certain types of paper grades. To expand the application of the long nip press to finer paper grades and newsprint, it will be necessary to develop a single-felted long nip press.
In a single-felted long nip press, the components in the nip would be listed in the following sequence: a smooth cylindrical press roller, the fibrous web or nascent paper sheet, the press fabric, the special belt, the lubricating film, and the arcuate pressure shoe. As implied by this sequence, the fibrous web actually contacts the cylindrical press roller in a single-felted long nip press.
In the double-felted long nip press, the water pressed from the fibrous web can pass into either of the two press fabrics, as well as into the grooves provided in the cylindrical press roller. In the single-felted long nip press, two of these sinks for water storage have been eliminated. There is only one felt and, because the fibrous web contacts the cylindrical press roller directly, this roller must have a smooth surface. Clearly, this loss of temporary water storage space must be made up in another fashion, because it is absolutely mandatory that voids be provided in the nip of the press, so that water pressed from the paper sheet in a single-felted long nip press will have a place to go.
Two approaches have been taken to achieve a satisfactory solution of this problem. Canadian Patent No. 1,190,779 shows a long nip press belt having voids on the felt side. These voids arise from the weave pattern of the base fabric and the absence of impregnation o the felt side of the belt, and provide a place into which liquid can be transferred during passage of the fibrous web, press fabric, and special belt through the nip.
The other approach is represented by the previously noted U.S. Pat. No. 4,946,731. This shows a long nip press belt having grooves on the side facing the felt, or press fabric, to store water pressed from the fibrous web.
While these two approaches were directed primarily toward achieving a solution to the water-removal problem in a single-felted long nip press, it has been found that us of either belt in a double-felted long nip press leads to enhanced dewatering.
The second of the these two approaches has proven to be the more viable, yet grooved long nip press belts are susceptible to a whole new set of problems. Common belt failures are caused by delamination and collapse of the grooves. In the case of the former, it is more accurate to say that the so-called land areas, which remain on the surface of the belt when the grooves are cut and which separate the grooves to some degree, peel from the surface of the belt in long strands. Not only does this leave the belt grooveless, but the long strands, the peeled land areas, also become entangled in the papermachine components. The invention shown in U.S. Pat. No. 4,946,731 provides a solution to the problem of delamination by anchoring the land areas to the belt with staple fibers from the spun yarns used to weave its base fabric.
The grooves may collapse under the elevated compression to which the belt is subjected in the nip. Here, it is more accurate to say that the land areas separating the grooves partially or completely flatten in the nip, closing the grooves to some degree, thereby reducing their effectiveness as a temporary storage place for water.
The manufacture of long nip press belts also presents difficulties. Typically, an endless base fabric must be provided in the dimensions required for a specific long nip press. That is to say, the endless base fabric must have a width, measured transversely thereacross, equal to that of the long nip press, and a length, measured longitudinally therearound, sufficient to permit the belt to be trained about all the tension and guide rolls on the press. As a consequence, each long nip press belt is to a large extent a unique item, making it difficult to streamline the process whereby belts of different size are manufactured.
The present invention includes a method for manufacturing long nip press belts of varying transverse and longitudinal dimensions, which does not require the provision of an endless base fabric in the dimensions required for a specific long nip press. As a consequence, long nip press belts may be readily and quickly manufactured for a variety of different presses on a single apparatus with common materials.
The present invention also includes a belt made in accordance with the method. The belt, in addition to its ease of manufacture, is also less susceptible to delamination and collapse of its grooves than the belts of the prior art.